Following on from Beginners Astronomy: Asteroids and Impacts
By Duncan Lunan
Like most people, until the 1960s I greatly underestimated the hazard from incoming asteroids and comets, and greatly overestimated the size that would be needed to cause serious harm. Both geological and biological evolution have indeed been punctuated by catastrophes, mass extinctions, followed by the rapid evolution of new forms; and the dinosaurs did indeed get their coup de grace at the time of a comet, or possibly an asteroid, which struck the sea off the coast of Yucatan. An early revelation came in Isaac Asimov’s article ‘The Rocks of Damocles’ (March 1966, reprinted in “Asimov on Astronomy”, Macdonald & Jane’s, 1974). Asimov demonstrated that a ‘city-buster’ asteroid could be as small in mass as a few dozen tons, about the size of a large desk if composed of nickel-iron. A two part article in Analog by Ralph A. Hall, (‘Secondary Meteorites’, January 1964) previously demonstrated that a similar object a mile across would sterilise a continent. Even then, although the crater would punch through the Earth’s crust to the magma, much of the energy would be radiated into space as the hole filled up.
But Worse Things Happen at Sea: tsunamis up to three miles high would radiate away from the impact, and though they’d become lower within a hundred miles, they’d rear up again in coastal waters at least to the thousand-foot height seen in Deep Impact. The pressure of the sea water outside would push the half-molten ringwall into the crater, and eventually, it would quench the rising magma; but not before huge amounts of water vapour, solid matter and energy had been released into the atmosphere, creating a storm which would cover at least an entire hemisphere while dust blanketed the whole planet. [J.E. Enever, ‘Giant Meteor Impact’, Analog, March 1966. The article had a cover by Chesley Bonestell showing an airliner caught in the winds drawing it into the firestorm. When Dr. Asimov revised ‘The Rocks of Damocles’ (‘Updating the Asteroids’, Fantasy & Science Fiction, August 1974), the Bonestell cover was a counterpart showing a cruise liner, on fire, borne on the crest of a tsunami 30 miles from the rim, with the air blast following and molten ejecta plunging around it.]
A comet nucleus ten miles across, or an asteroid eight miles across, would be as bad as the Chicxulub impact 65 million years ago which not only wiped out the dinosaurs, but made 90% of all species extinct. We don’t yet know whether that object was an asteroid or a comet, but if it was a comet, it was no larger than Halley’s, with a nucleus about ten miles across. The shockwave of its impact threw everything on Earth that wasn’t rooted to the ground ten feet up in the air. Our tiny mammalian ancestors fell unharmed and scuttled away, but the dinosaurs broke every bone in their bodies. Twenty minutes after the impact, the rain of molten ejecta had killed every living thing in North America; the mile-high tsunami had barely started across the Atlantic, and the pyroclastic flow riding on the air-blast was still blasting through Central America, but within the hour everything on the land surface of the Earth would be ablaze as the ejecta enveloped the planet. The crater was punched right through the crust into the magma, and as the magma rose, it met the sea falling in from above and vaporised it. The sea won, as it always does, but not before so much vapour and energy had been pumped into the atmosphere that the storm covered a hemisphere and the darkness was total worldwide for two years, during which the acid rain came down everywhere. 90% of all living species went extinct; but the little mammals in the burrows lived, because they could hibernate, they were nocturnal, and there was lots of carrion above ground to sustain them.
The Chicxulub impact may have been one of a series, and there have been many other destructive events before and since. Two within the last million years, both on land, coincided with reversal of the Earth’s magnetic field, and the late Carl Sagan suggested that was no coincidence. Since the crater of a large impact would punch right through the Earth’s crust, maybe the shockwaves radiating into the interior disrupt the currents which generate the magnetic field, and it’s a 50-50 chance which way ‘up’ it will regenerate. It would explain why the Great Dyings of marine species in Earth’s history seem to coincide with magnetic field reversals, although it’s not obvious why they would be affected by such results as temporary thinning of the ozone layer.
In the 1920s, C. Leonard Wooley discovered the flood layer at Ur in Mesopotamia (described in Ur of the Chaldees, Ernest Benn, 1929).
“During the seasons 1927-8 and 1928-9 our work on the prehistoric graveyard had resulted in the excavation of a huge pit some 200 feet across and between 30 and 40 feet deep… The shafts went deeper, and suddenly the character of the soil changed. Instead of the stratified pottery and rubbish we were in perfectly clean clay, uniform throughout, the texture of which showed that it had been laid there by water. The workmen declared that we had come to the bottom of everything, and at first, looking at the sides of the shaft, I was disposed to agree with them, but then I saw that we were too high up. It was difficult to believe that the island on which the first settlement was built stood up so much above what must have been the level of the marsh, and after working out the measurements I sent the men back to work to deepen the hole. The clean clay continued without change… until it had attained a thickness of a little over 8 feet. Then, as suddenly as it had begun, it stopped, and we were once more in layers of rubbish full of stone implements, flint cores from which the implements had been flaked off, and pottery… Taking into consideration all the facts, there could be no doubt that the flood of which we had thus found the only possible evidence was the Flood of Sumerian history and legend, the Flood on which is based the story of Noah… This deluge was not universal, but a local disaster confined to the lower valley of the Tigris and Euphrates, affecting an area perhaps 400 miles and 100 miles across; but for the inhabitants of the valley that was the whole world!”
The oldest surviving account of the Flood is in the Sumerian “Epic of Gilgamesh” and dates from c.2250 BC. (James B. Pritchard, ed., Ancient Near Eastern Texts Relating to the Old Testament, 2nd edition, revised and enlarged, Princeton University Press, 1955.) It’s the most detailed and clearly attributes the event to an impact. Versions of the legend are found in Egypt, the Hittite kingdom, India and China, and the Biblical one – which was picked up by Jewish exiles in the Babylonian captivity – is the only one which leaves out the impact. Not knowing that, Isaac Asimov nevertheless suggested that there had been one in ‘The Rocks of Damocles’. It began with “a cloud no bigger than a man’s hand” (a distant mushroom?), then a tsunami (“the fountains of the deep were unleashed”, and the Ark was carried inland to Ararat), and only after that the sky grew dark and “the windows of heaven were opened” with torrential rain (Genesis 7, 11). But the Sumerian account has a heat-flash, an incandescent rising cloud with ejecta (“the Annunaki lifted up their torches”), a ground-shock, an air-blast, and only then the tsunami and the deluge. Further off, Hittite legend says it was caused by the Moon falling to Earth (descending fireball), and the Egyptian account says it began with fire from the constellation Leo, while divine personages stalked the land striking down the populace with iron maces. The ancient Egyptians knew iron only from meteorites, and the Leonid meteors still provide spectacular displays every 33 years. It’s been suggested that parts of the story of Samson are a confused account of a Leonid fire-storm, although other writers associate him with Orion and its January meteors (Dr. Charles F. Herbeger, ‘Samson Strides the Skies’, Griffith Observer, March 1987).
The Henbury craters in Australia were formed around 2700 BC, corresponding to the origin of the Geminid meteor shower, which comes from an asteroid called Phaeton and has a high metallic content. The Flood could have been generated by one of those impacts, in the Persian Gulf: in 2354-45 BC there was an abrupt turndown in global climate, and there’s now evidence that the impact may instead have been in the Iraqi marshes, only a century before the Gilgamesh text (Mike Baillie, “Exodus to Arthur, Catastrophic Encounters with Comets”, revised edition, Batsford, 2000). It’s remarkable that one 19th century estimate put the date of Biblical Deluge at 2349 BC, though it’s probably a coincidence because other Bible studies then put it much further back (François Arago, “Popular Astronomy”, Longman, Brown, Green, Longman & Roberts, 1858). Around 2000 BC a so-called ‘bouncing asteroid’ (probably twinned, as many asteroids are) created a double crater at Campo Cielo in Argentina; this was the largest impact of modern times, with an energy release of about 300 megatons (Jeremy Hodges, ‘Somewhere Out in Space There Is an Asteroid…’, The Scotsman, 17th November, 1998). Drs. Victor Clube and Bill Napier believe these are part of an ongoing series of events from the break-up of a ‘super-comet’ in the inner Solar System around 3000 BC (“The Cosmic Serpent”, Faber, 1982).
In the earliest Sumerian account, Utanapishtim, the original Noah, tells Gilgamesh that he was warned of the coming flood by the god Ea speaking out of a wall. “After the flood, we shall return the people to their settlements.” But honesty compels Pritchard to note at the foot of the page that the word ‘after’ actually reads ‘before’, the word ‘return’ reads ‘withdraw’, and the word ‘to’ reads ‘from’. In other words, the true translation reads “Before the flood, we shall withdraw the people from their settlements” – it’s a warning and an evacuation, not a divine punishment at all. In “Man and the Stars” I had some fun with the Ark, which was symmetrical, sealed, two-decked and unsteerable, with a hatch in the top which you couldn’t see out of and a door in the side which could only be opened once it was on land. Obviously, it’s a larger version of the Apollo capsule…
Alan Bond and Mark Hempsell believe they have evidence that a low density 1-km rock asteroid with a mass of 800 million tonnes passed over Sumeria at 14 km/sec on June 29th, 3123 BC, clipping the Gamskogel ridge and impacting the Köfels area in the Austrian Tyrol, triggering a massive landslide which erased the main crater, while smaller fragments caused other impact features in the area. “As the object travelled up the Adriatic Sea… and across the Alps the supersonic shock would have caused considerable destruction on the ground beneath the trajectory. The impact… would release energy equivalent to 1.4 x 1010 tonnes TNT. [The plume] would rise… to some 900 km before falling over the Levant and Sinai causing considerable destruction over a wide area… There would have been many direct casualties, near 100% mortality over areas of thousands of square kilometers in both the Alps and the Near East. There would also have been a severe global climate change that caused further death and social disruption.” [Bond, A. and Hempsell, M.: A Sumerian Observation of the Köfels’ Impact Event. Alcuin Academics, York (2008).]
It’s been suggested that a meteor airburst, like the one over Siberia in 1908, might have caused the conflagrations in New Zealand, which wiped out the flightless Moa, contemporary with the first human settlement and the second wave on Easter Island c.1200 AD. (Duncan Steel & Peter Snow, ‘The Tapanui Region of New Zealand: Site of a “Tunguska” Around 800 Years Ago?’, in A. Harris & E. Bowell, eds., “Asteroids, Comets, Meteors 1991”, Lunar & Planetary Institute, Houston, Texas, 1992.) But it’s not generally realised that there were three Tunguska-scale events in the 20th century alone – as it happens, none were over populated areas and the ones in South America and the North Pacific attracted little attention. Tunguska-type objects are in the same class as fireball meteors, dust balls which disintegrate before reaching the ground, producing mainly blast damage. But in 2002 a dense object, probably nickel-iron, flew up the spine of North America and left the atmosphere again. Had it struck it could have devastated an entire state, and started a chain of much worse events: Prof. Colin McInnes has written a short but telling piece imagining the nuclear consequences of a similar event on the border between India and Pakistan.
My book Incoming Asteroid! What could we do about it? (Springer, 2013) was the climax of a ten-year discussion project on the question, ‘If we knew there was going to be an impact in ten years’ time, what could we do? And what would we do?’ We had to choose a ‘designer hazard’, a Goldilocks object which was neither too easy nor too difficult to deal with, and we settled on a 1-kilometre rock in an orbit with a 3.3 year period, similar to Encke’s Comet and to several Earth-grazing asteroids. With current technology and a massive international effort, it could be diverted using light-sails, mass drivers, nuclear weapons, impactors, high-energy lasers, and probably a combination of all of them. Nearly all threatening asteroids of that size and above have been detected, with no immediate hazards, and new telescopes and satellites coming on line should lower the detection threshold to 100 metres – still enough to cause serious damage. But in the next ten years all the likely impactors down to city-buster size should be identified and tracked, and if we can find them all in time to prevent a disaster, then we will have been very, very lucky.
Incoming Asteroid! What could we do about it? By Duncan Lunan is available in print or as an e-book, from Amazon, through bookshops or from the publishers.
Related articles: The Sky Above You – October 2021
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